In recent years, a semiconductor memory device such as NAND flash memory is mounted on many electronic devices. Along with a demand for multifunctional electronic devices, an increase in storage capacity is needed for the semiconductor memory device and correspondingly miniaturization of memory element is needed.
A NAND flash memory typically employs a MOSFET memory transistor in which a floating gate and a control gate are stacked. For the NAND flash memory, a NAND cell unit in which such memory transistors are connected in series is configured. One end of the NAND cell unit is connected to a bit line via a select transistor and the other end thereof is similarly connected to a source line via a select transistor.
With the development of scaling down, there is known a NAND flash memory in which a dummy memory transistor is connected between a memory string and a select transistor. The dummy memory transistor has substantially the same structure as a typical memory transistor but is not used for storing data.